Thread: Some contemplations on overdrive

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Well, guess I'm not much into theory...but I do know the 5 speed with the 4.11's works great for me, super acceleration around town and playing with the boys, and a very nice 2200 rpm cruise down the highway when travelling... If there's a transmission and gear ratio combination that will work better doing both things, I'd probably buy it....

Originally Posted by R Pope

An example of what Willowbilly is saying , My cousin has a Kenworth with a 425 Cat and 15 over trans., 4.11 gears. My Eagle has a 425 Cat and 15 direct, 3.55 gears. RPM at 60 mph is 1750 in both. Speeds in all gears is the same. The OD trans is a bit noisier in top gear, and the direct geared one seems to pull a hill better in top gear. On a long uphill grind, the Kenworth can go down to Direct and pull like crazy, quieter than the IHC in 14th (reduced gear). Once you shift them both down below that, there's no difference. Neither gets more than 5-6 MPG grossing 120,000 lbs, so I can't say if the reduced friction makes any difference in fuel consumption. Logic would say it should, but a few percent at that high rate would be awful hard to detect.

If there were a milage difference in real life I too doubt it would be measurable on a big rig.
Thanks for the real life comparison. That is the kind of information I was hoping to hear more, pro or con.

Here is some info on the 700R-4 The good and Bad . http://www.smokemup.com/tech/700r4.php

I would love to design a car with a hydraulic drive someday. Unfortunately I do not have the requisite knowledge of hydraulics to engineer it myself.

My theory is to make the ultimate 'dancer'. have a nice fat hydraulic pump direct mounted to the engine. have hydraulic motors on the rear wheels, and hydraulics on all four corners.

When they make 'dancers' today, it requires a TON of lead in the trunk to run the electric pumps. My idea would eliminate the extra weight of the batteries, and ( should ) allow the vechicle to do more.

I do know enough about hydraulics to see some obvious design problems: first, the hydraulic drive would be a parasitic power loss. The fluid would heat ( power loss ) and any time you transfer power from one type to another ( mechanical - fluid - mechanical ) you suffer power loss. It takes power to convert power types.

BUT.. is seems to me that if you started with a variable flow pump, match your hydraulic motors correctly, you would have one SMOOOOTH acceleration.

I think there has been some big moves toward a hydrostatic car and more efficient transfer. One big advantage is keeping the engine at a constant rpm, which would allow you to just set right at the peak of the torque curve. Snowmobiles do this quite well.
The big mine trucks use electricity. The diesel runs a generator and the wheels have electric motors that are also retarders.

You guys think about all the angles! Since a fluid does not compress, why not build a drag car with the engine set to run at max torque (after warm up) and put the power through to the drive wheels hydrostatically, using a higher volume pump than the motor, and use a positive control valve controlling bypass, linkaged to the pedal on the right, you could theoretically turn the rear tires 300 + mph as soon as you got the go light! can you imagine the flying parts? Seriously though, I wondered about the logic of gear ratios as a youngster when I discovered that sprocket ratio on a bicycle was affected by the throw length (stroke) on the pedal cranks. Am I getting it?

Originally Posted by JoelGrimes

You guys think about all the angles! Since a fluid does not compress, why not build a drag car with the engine set to run at max torque (after warm up) and put the power through to the drive wheels hydrostatically, using a higher volume pump than the motor, and use a positive control valve controlling bypass, linkaged to the pedal on the right, you could theoretically turn the rear tires 300 + mph as soon as you got the go light! can you imagine the flying parts? Seriously though, I wondered about the logic of gear ratios as a youngster when I discovered that sprocket ratio on a bicycle was affected by the throw length (stroke) on the pedal cranks. Am I getting it?

I think there has been an issue of parasitic loss in the hydrostatic drives. But in theory if you could put the engine at the peak of the torque curve and vary the multiplication to keep it at peak that would be a good thing.
You bicycle analogy is a good simplification of crankshaft stroke.

FWIW Ford does have a constant variable transmission in a few of it's smaller cars. ZF builds it.

A poster on another board offered this. This is the kind of stuff I was looking for.

You knew this was going to happen. I had to put some numbers behind this discussion.

A typical helical gearset is 98-99% efficient. In a direct drive top gear there are none, in an OD top gear there are two. For the sake of discussion, assume 99% for the input shaft gearset and 98% for the OD gearset. These assumptions are supportable given that the input set is typically quite robust and has a relatively low reduction whereas the OD gearset is typically rather weak and is being used inefficiently as a speed increaser.

A hypoid bevel gearset is 92-96% efficient; efficiency decreases as ratio increases. For the sake of discussion assume 95% for a 3.08 gearset and 94% for a 3.55. These are supportable assumptions given the range of 1.5:1 to 6:1 normally encountered.

A Ford 300 at 2000 RPM produces approx. 130 HP under full load and WOT at a BSFC of approx. 0.5 lbs/HP-hour. At cruise under some unspecified but significant load assume for arguments sake that it produces half the HP (65HP) and that the BSFC is 10% worse or 0.55 lbs/HP-hour. Assume further that the gearing is such that this occurs exactly at 60 MPH.

If this engine were to be coupled to the direct drive transmission with the numerically lower rear end, it would deliver:

65 x 0.95 = 61.75 HP to the rear wheels.

It would consume:

65 x 0.55 = 35.75 lbs of fuel per hour which is also 35.75 lbs of fuel per 60 miles

At 6.25 lbs per gallon this would be:

5.72 gallons per 60 miles or 10.49 MPG, arguably what one could expect with a heavy load.

If this same engine were to be coupled to an OD tranny with a numerically higher rear end, to produce the same 61.75 HP at the rear wheels, it would need to produce:

61.75 HP / (.94 x .98 x .99) = 67.7HP at the flywheel

That would result in a consumption of:

67.7 x 0.55 = 37.23 lbs of fuel per hour which is also 37.23 lbs of fuel per 60 miles

At 6.25 lbs per gallon this would be:

5.96 gallons per 60 miles or 10.06 MPG.

So running at highway speeds under a significant load, the difference would be 0.43 MPG. At lighter loads it would be proportionally less, meaning about half as much difference at cruise running empty or about .20-25 MPG.

This treats only the effect of transmission losses and ignores all the other sources of inefficiencies such as bearing losses, seal drag aero and rolling resistance, etc. assuming that they are equal in both cases.

Obviously YMMV both literally and figuratively.